There has been known a display device that automatically adjusts brightness of a light-emitting display such as a fluorescent display tube in accordance with ambient luminance. For example, as a head-up display installed in a vehicle, there is such a device that detects a luminance in front of a vehicle that becomes a background of a display image projected on a windshield of a vehicle, with a light sensor, controls the brightness of a display image projected on a windshield in accordance with the detected luminance in front of a vehicle, increases the display brightness when the luminance increases (brighten) and decreases the display brightness when the luminance is lowered (darken), and adjusts the display brightness to be optimized for the ambient luminance (see Patent Literature 1, for example).
In such an automatic adjustment of a light-emitting display, a temporary and sudden change may occur in a luminance value detected by a light sensor, in cases where external light comes in some places while a vehicle is traveling or a wiper is working. This causes a problem that a change in the display brightness is repeated in a short period of time and a display flicker occurs, in the case of adjusting the display brightness in accordance with a noise component of such a detected luminance value. In contrast to this, the Patent Literature 2 discloses a display device that performs determination of a luminance change, twice or more based on a detected luminance value, and adjusts a display brightness based on the latest luminance detection value, when it is determined that there is a luminance change in a predetermined successive number of times.